Numerous connections have been made between the generation and presence of oxidative free radicals in brain tissue and neurological disorders. For example, 1) Jenner (26) links oxidative stress to Parkinson's, Alzheimer's and Huntington's diseases. 2) Recent clinical studies have demonstrated that alpha-tocopherol (vitamin E) and selegiline (deprenyl), pharmacologic agents that have antioxidant activity, can slow the progression of moderately severe Alzheimer's disease (27). 3) Antioxidants such as vitamin C and vitamin E may have an important role in the treatment of diseases whose pathogenesis involves free radical formation and impaired antioxidant defenses in the aging population. Oxidative damage has been hypothesized as central to the neurodegenerative processes such as Alzheimer's disease (28). According to the free radical hypothesis, Alzheimer's disease is an acceleration of the normal aging process in affected brain regions which become progressively more damaged by free radicals generated from metabolism. In Alzheimer's disease, the cerebral cortex seems to have increased antioxidant requirements, increased sensitivity to free radicals, and levels of the free radical defense enzymes, such as superoxide dismutase, that are reduced by 25-35% in the frontal cortex and hippocampus. The loss of hippocampal cholinergic neurons is a key feature of Alzheimer's disease and these neurons seem particularly vulnerable to the deleterious effects of free radicals on the muscarinic cholinergic receptor (29). 4) Antioxidants have been tested as drugs for Parkinson's disease (30), and it was found that selegiline, which may act as an antioxidant since it inhibits oxidative deamination, delays the onset of the disability (31). 5) Peyser et al. concluded that antioxidant therapy may slow the rate of motor decline early in the course of Huntington's disease (35). 6) According to Challem (32) free radicals and oxidative stress may be factors involved with the pathogenesis of Mad Cow disease. 7) The oxidative modification of low-density lipoprotein (LDL), termed lipid peroxidation has been shown to be an initiating event in atherosclerosis. Probucol, an antioxidant, is effective in reducing the rate of restenosis after balloon coronary angioplasty (36). Oxidized LDL has several detrimental effects on cells including brain cells such as cytotoxicity and vascular dysfunction.
Therefore, increasing the concentration of free-radical scavengers or antioxidants in brain tissue may provide therapeutic benefits to subjects suffering from neurodegenerative diseases. Sano et al. conclude (27) that the use of the antioxidants, selegiline or vitamin E may delay clinically important functional deterioration in patients with Alzheimer's disease. Their results are particularly significant because vitamin E does not cross the blood-brain barrier in large amounts, and still it has a measurable effect.
The enhancement of the antioxidant potential is useful in treating of many diseases. For example, the increase of antioxidant potential achieved by this invention will be able to treat stroke and neurovascular diseases. It is known that ischemic stroke is the most common neurologic disorder causing death or disability among adults. Strokes of all types rank third as a cause of death, surpassed only by heart disease and cancer. Ischemic stroke events account for approximately 85% of all strokes. Because no medical or surgical treatment has yet been established as reversing the effects of acute ischemic stroke, early identification and treatment of persons at the time they present with stroke is compelling, if such a treatment is efficacious. Currently, there are no approved treatments for stroke. The damage from stroke is caused by occlusion of a vessel, thereby restricting the delivery of oxygen in the blood to an area of the brain. Much of the damage is caused by damage from oxygen free radicals in the area served by the occluded vessel after reperfusion of the affected area (37). Thus, increasing the antioxidant potential of the brain may have beneficial effects on stroke and other neurovascular diseases.
Therefore, increasing vitamin C concentrations in the brain by providing dehydroascorbic acid to the subject could enhance antioxidant potential in the central nervous system and may be therapeutic in stroke and neurovascular diseases as described.
Researchers have proposed that atherosclerosis, and its deadly effects of heart attack and stroke, develops in relationship to oxidation of low-density lipoproteins (LDL) carrying cholesterol in the blood. The theory states that free radicals generated by the body's own immune cells oxidize LDL which is taken up by cells of the vascular intima initiating the atherosclerosis lesion. Ultraviolet and gamma radiation, cigarette smoke and other environmental pollutants, also cause oxidative damage to cells and vital compounds. The damage leads to the development of several chronic diseases including cancer and coronary heart disease (CHD). It was further proposed that antioxidants such as vitamin E and C and the carotenoids could prevent damage and the ensuing diseases. Many epidemiologic and animal studies have offered evidence to support the theory (33, 34). Recent studies demonstrated that the antioxidant proburol is effective in reducing the rate of restenosis after balloon coronary angioplasty (36).
Evidence suggests that the neuropathology of Huntington's disease, a neuropsychiatric disorder, results from excessive activation of glutamate-gated ion channels, which kills neurons by oxidative stress. It was reported that antioxidant therapy may slow the rate of motor decline early in the course of Huntington's disease (35).
Vitamin C enters cells, in vitro, through the facilitative glucose transporter GLUT1 in the form of dehydroascorbic acid and is retained intracellularly as ascorbic acid (1). In order to test the hypothesis that GLUT1 transport of dehydroascorbic acid is a primary physiological mechanism for tissue acquisition of vitamin C, we investigated the transport of vitamin C across the blood-brain barrier (BBB) in rodents. GLUT1 is expressed at the BBB on endothelial cells and is responsible for glucose entry into the brain. Ascorbic acid, the predominant form of vitamin C in blood, was incapable of crossing the BBB while dehydroascorbic acid readily entered the brain and was retained in the form of ascorbic acid. The transport of dehydroascorbic acid into the brain was competitively inhibited by D-glucose, but not by L-glucose. These findings define the transport of dehydroascorbic acid by GLUT1 as the mechanism by which the brain acquires vitamin C, and point to the oxidation of vitamin C as the important regulatory step in the accumulation of the vitamin by the brain.
Dehydroascorbic acid, the oxidized form of vitamin C, was previously found to be transported through the facilitative glucose transporters. Expression of GLUT1, GLUT2, and GLUT4 in Xenopus oocytes conferred the ability to take up dehydroascorbic acid which was retained intracellularly after it was reduced to ascorbic acid (1). It was also established that facilitative glucose transporters are involved in the transport and accumulation of vitamin C by normal human neutrophils and the myeloid leukemia cell line, HL60 (1-3). In these cells dehydroascorbic acid is transported across the cell membrane and accumulated in the reduced form, ascorbic acid, which is not transportable through the bidirectional glucose transporter (1-3). Ascorbic acid may be transported through a Na.sup.+ -ascorbate co-transporter that is reported to be present in small intestine, kidney and adrenomedullary chromaffin cells (4). The co-transporter has not been molecularly characterized and no Na.sup.+ -dependent ascorbic acid uptake in white blood cells has been found (2,3).
GLUT1 is expressed on endothelial cells at the BBB and is responsible for glucose transport into the brain (5,6). In the 1880's, Ehrlich found that intravenously injected aniline dyes colored all of the organs of experimental rabbits except the brain and the spinal cord (7,8). This observation led to the eventual discovery that the BBB is comprised of a wall of capillaries forming an endothelial barrier between the blood and the brain, functioning primarily to regulate the transport of nutrients and waste products (9,10). Several nutrient transporters have been identified at the BBB including GLUT1, a monocarboxylic acid transporter, neutral amino acid transporter, amine transporter, basic amino acid transporter, nucleoside transporter, and purine base transporter (11). Here it is shown in rodents that vitamin C crosses the BBB through GLUT1 only in the oxidized form, dehydroascorbic acid, and is retained in the brain in the reduced form, ascorbic acid.
The present invention allows for the controlled introduction of the antioxidant vitamin C into brain tissue, which should serve as an important therapeutic method to treat and prevent various disorders associated with free radicals and oxidative damage.